Ryan Greene, Trevor Dunn
Trevor Dunn, M.F.A. | College of Arts and Sciences | Department of Art, Art History and Design
My research focused on constructing a wood-fired train kiln. I utilized ashlar fine tooled masonry techniques to create a dry-stacked formation. Primarily, I analyzed the different materials to which I had access and determined the proper steps to create an ideal structure. In order to accomplish this, I sorted, cut, and shaved various series of both insulating and dense fire bricks to create a perfect fit, an expansion joint or a level surface. If a level surface could not be attained using solely bricks then a compound termed “butter” was applied, but I utilized a high temperature refractory mortar when I required a brick to be held in place. Furthermore, arches are formed to span the door, chamber, firebox and throat of the kiln using skewback along with varying arch brick, and subsequently, a steel frame is used to buttress the arches as well as hold the bricks in place during expansion; for this reason, I ascertained the importance of inspecting the structure to be true, level, flush, and plum. Through my research, I determined it is of the utmost importance one understands everything about their materials on hand; this allowed me to design a kiln to fit together in both a tight and level manner atop an unlevel foundation. I found that the efficiency of building a dry-stacked kiln is reliant upon organizing bricks according to their differences in height by sixteenths of an inch. In addition, a brick saw and several levels must be used with precision accuracy.
Hello, my name is Ryan Greene, and today, I will be speaking about my research on constructing a wood-fired train kiln using ashlar fine-tooled masonry techniques to create a dry stacked formation. I will discuss the different steps I took to both plan and create the structure beginning with the foundation and continuing on to the arches and chimney. Furthermore, I will discuss the reasons for utilizing different types of materials throughout the kiln.
Before beginning the build, my professor, Trevor Dunn, and I spent around three months creating detailed blueprints and materials lists for the build. We contacted refractory distributors to determine the best cost for the product received, and we re-designed the kiln based on the different materials a distributor had on hand. Additionally, Professor Dunn created a computer animated design (CAD) rendering, and this allowed us to break down the kiln into parts to view digitally or on print outs. Originally, the concrete slab we built the kiln upon was off level by one and three-quarters inches over the span of nine feet. In order to correct this, I applied mortar below the concrete masonry unit partition blocks (CMUs), and I utilized a snap level on a length of string to ensure they were level.
Following the foundation, we cut sheets of cement board to span the gaps between the CMUs and laid the first floor of insulating fire brick (IFB). These bricks were sourced from a pallet of scrap bricks as well as retired electric kilns, so their dimensions were irregular. For this reason, some bricks were cut to fit while others were packed with a compound termed “butter”—a mixture of fire clay, sand, and water—after being laid. The second floor is comprised of dense fire brick because of their higher resistance to corrosive minerals within the wood. Due to their resistance, dense fire brick line the entire interior of the kiln—the hot face. In our kiln, we utilized thirteen-and-a-half inch 60D bricks in our second floor, for their larger size is superior for anchoring the first floor in place, and their unique taper allows for a tighter fit. It is essential that this floor is perfectly level before the walls are laid.
The first course of the walls is the footprint for the chamber, firebox, and chimney of the kiln. First, IFB are laid around the perimeter with dense fire brick on the hot face of the wall. The IFB and dense fire brick are laid parallel, so they are independent of each other and do not form a structurally sound wall. As a result, a header course of only dense fire brick is laid perpendicular to the walls below to tie the walls together. There are two header courses within this kiln (every fifth course), and the second header course is level with the castable atop the arches.
Arches are used to span the loading door, chamber, firebox, and throat of the kiln, and they are created with varying arch brick along with skew brick and arch forms. Arch bricks vary in their taper and are categorized as either a one, two, or three arch brick with a width dependent on the series. First, arch bricks are laid on a flat board to determine how many of what type of arch brick are needed to span the correct distance. Once the span is correct, we verified the skewback was equal on both sides and traced the rise of the arch onto the board. This is then used to create an arch form and skew brick, and skew brick are cut on a brick saw using a jig to ensure consistency. Arch forms are shimmed to the height of the skew brick; arch bricks are then added on each side simultaneously as a means of maintaining equal weight distribution. The final arch brick is called the key brick, and the arch must be buttressed before the key brick can be set. For the loading door and throat arch, coddle boards are put in place around the arch to make a form for the castable. The top of the form is made level with the course immediately above the rise of the arch, so the next course or bricks can be laid. The chamber and firebox arch do not require castable, for no bricks are laid above them; although, these arches are buttressed by steel, so IFB are cut and used as an insulating spacer behind the skew brick which prevents the steel from warping.
Prior to building the firebox arch, the firebox itself is a very meticulous component. The grate bar slots, hob, and primary airs are all included within the firebox and require bricks of specific dimensions. The grate bars and hob create an elevated ledge for the wood to burn upon, and this elevated firebox is what makes a train kiln distinct. Above the grate bars, the primary airs are a system of nine port holes which allow us to control how much oxygen is allowed into the kiln. IFB are used to plug the port holes, so we make the ports a sixteenth of an inch larger on all sides to allow the IFB to slide in and out. To do this, a small amount of high temperature refractory mortar is used to raise the bricks around the ports; consequently, the top of the firebox must be re-leveled using “butter” before the arch is constructed.
By this point, steel has been prepared and welded into place on the kiln. To prepare it, we grinded the steel to remove rust and applied phosphoric acid to make the iron oxide inert. A high temperature paint is used to further protect the metal from oxidation; although, the paint is grinded off to expose the metal when welding and repainted after. The steel acts as a frame and buttress for the bricks and prevents them from moving as they expand and contract during firings, so it is required regardless of whether the structure is dry stacked or built using mortar.
Overall, I have made one all-encompassing discovery through my research of ashlar fine-tooled masonry—it is imperative that all bricks are sorted according to their size differences by sixteenths of an inch. The concept of knowing your materials is of far greater importance when attempting to dry stack, for the bricks must fit perfectly together without the use of mortar or “butter” to account for inconsistencies. Dense fire bricks of the same series are often inconsistent in their dimensions; also, the types of materials used varies throughout the kiln.
The primary challenge begins with the process of manufacturing dense fire bricks. It involves firing them to extreme temperatures; consequently, the bricks expand and contract as they absorb heat and cool at different rates depending on their location within the kiln. Kilns often have hot spots—areas which reach higher temperatures than others, so the different rates of heating result in up to an eighth of an inch difference above and below their expected dimensions. For example, a brick expected to measure three inches high could measure between two and seven eighths inches and three and one eighth inches. If larger bricks are used on one side of the kiln than the other, then the top of the walls is no longer level. This can be used as an advantage, although, when it is already unlevel. For this reason, I measured and sorted the bricks into categories with increments of sixteenths of an inch. I used bricks of the same size on each course, and each course’s brick size could be independent from the next. Furthermore, the manufacturing process also causes a slight concave warp due to pyroplastic deformation. All bricks warp slightly while others warp more greatly, so we typically place the curve down and sort out the more greatly curved bricks. These are used to bridge small bumps along the wall or are cut into half bricks to reduce the curve. On the other hand, IFB are not fired to as high of a temperature, so their dimensions are extremely consistent.
Other than consistency of materials, the types of materials used varies throughout the kiln. We constructed this kiln using fourteen different types of bricks, four types of bonding or leveling compounds, and six different types of steel. For the walls of our kiln, we used three-inch series IFB and three-and-a-half-inch series dense fire bricks, so we built up one more course of IFB and shaved it level with the dense fire brick for the header courses. The header course was built with nine-inch series 60Ds because their taper locks the bricks in place and creates a perfect fit for the spy port. In our foundation, the CMUs were leveled with mortar which was composed of Portland cement; however, this is not suitable for high temperature construction. As a result, the castable and mortar used within the kiln are composed of a calcium aluminate cement to resist higher temperatures. Moreover, the aggregate within the CMU mortar and castable was coarser while the high temperature mortar and “butter” had a much finer aggregate. Within the “butter,” sand acts as the aggregate and there are sands of different mesh to choose from. Play sand worked very well for us while sixteen mesh sand proved to be too coarse to feather out. In most parts of the kiln, a single grain of sand could throw you off level since no “butter” or mortar was used, yet the bricks are shipped with a coating of sand to protect them. This required every brick to be cleaned off to ensure the structure remained level. The different materials all require differing methods of preparation and handling, so it is vital one remains conscious of the different applications for each material.